Powder metallurgy



Jan. 14,1941, HAR Y 2,228,600

" POWDER METALLURGY Filed Oct. 5. 195a INVENTOR ATTORNEYS PATENT OFFICE lurgy and particularly with promoting difiusion oif metals across grain bmmdaries ,in ametal .a UNITED-J STATES rownna nmrannnacr (lharles Hardy, Pelham Manor, N. Y., assignor to- Hardy Metallurgical 00., New. York, N. Y., a corporation of Delaware Application mm- 5, 1933, Serial No. 233,387

Claim. This invention is concerned with pcwdermetalpOwder aggregate or brique'tteflwith a view to trial hazards, especially when cyanides are em- (Cl. 75-22) r 'ployed, and is likely to'result in contamination of the product. The immersed briquette almost always has a certain degree of porosity and the molten material of the bath tends to impregnate tortion, even though the briquette be relatively 6 bonding the grains together. The invention aims the pores. Removal of the impregnant from the to shorten the time necessary to accomplish such briquette is diflicult and, if it is allowed to remain, bonding and to bring it about with expenditure of frequently causes sweating, corrosion, etc. Moreless energy than employed heretofore. The invenover, the problem is not merely one of raisin the i tion also aims to'reduce distortion of powder metal temperature of the brlquette rapidly. 'Even' after 10 briquettes during the bonding operatic the briquette has been raised to an appropriate 10 In powder metallurgy it is customary to press temperature, the 'difiusion of metals across the a body of meta-l powders, with or without a binder, grain botmdaries in heretofore customary practice to form a coherent massor briquettte. -.The body is slow. Frequently itis necessary tomaintain the usually is compressed in a relatively .cold condibriquettes at an elevated-temperature for several tion in a mold, and thereafter the resulting days. Under such circumstances, heat consump- 1 briquette is removed from the mold and subjected tion is high, furnace capacity is low and consumpto-heat treatment in a relatively unsupported or. tion of gas such as hydrogen or nitrogen emunconfined condition, in order to bring about a played to maintain a protective non-oxidizmg or bonding of the metal grains or particles which ,reducingatmosphere is excessive and costly. form the briquette. The briqiiette may be heated In the course of my investigationsI have found 20 to the. point of incipient fusion sothat grain that generally speaking, the lower the melting boundaries are destroyed, but under such condipoint of a metal the lower is'its rate of diifusion tions great distortion of the unsupported briquette in the solid statev under the influence-of heat at is likely to occur. "Distortion of an unsupported ordinary pressures. Thus, briquettes of lead or tin brlquette generally is less if incipient fusion is powder must be heated for days to bring about 25 avoided and the briquette'merelyheated to a reladiilusion welding between the powder particles tively high temperature but below its melting unless a high d ree of pressin'e is employed point, under which-conditions metal tends to difsimultaneously. Aluminum and copper difiuse fuse across'grain boundaries while the grains inthesolidstateatahigherratethanleador'tin,

remain in a relatively solid state. However, this but still relatively slowly. Metals having a very 30 procedure (which is known as diffusion welding) high melting point mien as tungsten tend. to

. is relatively slow and in many instances must be diffuse relatively rapidly at temperatures in the prolonged for 'severalda'ys in order to secure an neighborhood of 2000" C. but below their melting adequate bond between particles. The longer points. Certain metals, such, for example, as

treatment time afiords an opportunity for some chromium, appear at first to be exceptions to the 5 distortion to occur. Hence, difiusion welding does above enunciated rule, because diifusio'n from not eliminate distortion. Since powder metallurgy grains of chromium under the influce of heat finds its principal application in the manufacture ordinarily is very slow. This, however, appears of articles of accurate configuration and competes to be due to' the skins of oxidation products on 40 with die-casting processes and the like, distortion the chromium grains and-diimsion is fairly rapid 40 even when relatively slight presents a serious when care is taken to use chromium powder hav- 1 problem. ing absolutely clean surfaces. V

As indicated above, diffusion welding as em- As a result of my investigations I have discovployed heretofore in powder metallurgy is a slow ered that internally inducededdy currents of high process. The briquette usually is heated by introfrequency (1. e., ofafrequency of at least 50,000 45 ducing heat from an exterior source, for example, cycles per second) not only bring about a rapid by placing a briquette in a furnacein the presence rise in temperature of a briquette of metal powder, of a gaseous atmosphereor solid packing. Under but also, it maintained in the briquette after an such conditions a long time is consumed in bring- I appropriate temperature is attained stimulate ing the briquette upto the necessary temperature. greatly the rate at which diffusion welding occurs, 50

- A more rapid rise in temperature results when a so that strong metal bodiescan be formed from briquette is immersed in a molten bath of metal metal powders in but a small fraction of the time or a thermally stable salt, such, for example, as a and with but'a small fraction of the energy concyanide. Such-heating, however, involves indussumed in prior practices and with minimumdisunsupported. These results appear to be due to two factors: (1) an actual carrying of metals across grain boundaries in the briquettes by the eddy currents and (2) a tendency for the eddy currents to cause non-metallic impurities in the briquettes to ball up" or coalesce and so to reduce the resistance to migration of metal across the grain boundaries. As will appear more fully hereinafter, the operation of these factors in the process of my invention is confirmed by comparative energy measurements and by microscopic examination of specimens which have been made by the process. But whatever be the explanation, the fact remains that I have succeeded in accelerating markedly the rate of diffusion in an apparently solid state of even such low melting metals as tin and lead and such relatively high melting point metals as chromium, which heretofore have required both long time periods and much energy for diffusion welding. When the briquette is formed of a single metallic constituent, either a metal or an alloy, bonding of the particles proceeds under the influence of the internally induced eddy currents at a temperature below the melting point of the constituent but pref erably in excess of two-thirds of the temperature at which melting would occur. To take a concrete-instance, tin melts at 232 C., and in the I practice of my invention a briquette containing no other metal than tin preferably is subjected to internally induced high frequency currents while it is maintained at a temperature ranging from about 154 C. to slightly below 232 C. for a short time, during which a thorough bonding of .the particles results. When the briquette to be treated contains a plurality of metallic ingredients, for example 20% by weight of particles of elemental nickel (melting point 1452 C.) and by weight of particles of elemental iron (melting point 1530" 0.), it is subjected to the internally induced eddy currents while maintained at a temperature below the weighted average of the melting points (in this example 1514 C.) but above a temperature that is about two-thirds of such weighted average (in this example 1010? C.). Under such conditions alloying and bonding proceed almost instantaneously, and the treatment time becomes a matter of minutes as compared with hours or days in heretofore customary practice. Accordingly, the acceleration of difiusion welding in a coherent mass of metal powder by internally inducing eddy currents of high frequency within the coherent mass while maintaining it at a relatively elevated temperature (so that diffusion welding tends to occur) but below the weighted average of the melting points of its metallic constituents, and preferably below the melting point of any of its metallic constituents but above a. temperature that is about two-thirdsof said weighted average. '(In the case of a briquette containing only one metalllc constituent, the weighted average" referred to above is, of course, its own melting point.) In most-instances, because of the heat economies obported, suffer a minimum distortion.

In the practice of my invention, superior results are obtained by placing the aggregate or briquette ina field of a high frequency current so that currents of' correspondingly high frequency are in-- my invention contemplates duced within it. Metal powder briquettes thus treated rapidly attain high temperatures. 1 attribute the rapid heating to the fact that a myriad of eddy currents are set up in the individual particles or small and perhaps partially insulated groups of particles. In this way heat is generated throughout the powder metal body and no time is lost in conducting heat into the mass. Rapidity of heating, however, does not explain completely the' fact that diffusion is accomplished more rapidly in the practice of my invention and, as indicated hereinbefore,,I believe that the currents induced within the mass actually carry metal from one particle to another (even though the mass remains at a temperature below the melting point of any of the particles), and thus aid in diffusion and bonding. In any event, the process of my invention by accelerating and improving diffusion across grain boundaries in briquettes of metal powders produces objects which are comparable in hardness, malleability, tensile and compressive strengths, and other desirable properties with those obtained heretofore by other methods such as casting, and does so in but a mere fraction of the time and at a mere fraction of the cost of prior powder metallurgical processes. My invention is particularly advantageous in the treatment of briquettes containing metals which are difilcult to bond by diffusion welding either by reason of an inherently low'rate of difiusion as in the case of 3 lead and other low melting point metals or by reason of skins of oxidation products as in the case of chromium. To illustrate, a compressed mass of iron, nickel, and chromium powders may be heat treated to produce excellent stainless steel in from 2 'to 4 hours by inducing high frequency currents in-the mass, whereas equivalent treatment by conducting the heat into the mass from an outside source requires in the neighborhood of 96 hours.

Powder metal aggregatesor briquettes formed by compressing a loose mass of the powders in a relatively cold state are not homogeneous. Individual powder particles may be distinguished in the aggregate, which has a granular structure. 45 The grain boundaries are characterized by the presence of films of occluded or adsorbed gas or by solid oxidation products. The heterogeneous character of the mass is probably responsible for the intensity of eddy currents induced therein. 50 In commercial practice, metal powders almost inev'itably are contaminated at their surfaces by such adsorbed oroccluded gases or by thin films of oxides, carbonates, sulphates, etc. Consequently, it is, in general, unnecessary to actually 55 add an insulating media to assure the desired heterogeneous structure. However, no attempt should be made to remove completely films of occluded or adsorbed gases from the metal particles which form the briquettes and, in certain instances, it may be desirable to assure that the particles are not too clean and to employ small amounts of a binder of insulating character to assure adequate intensity of induced high frequency eddy currents. The presence of aconsiderable proportion of foreign material at the grain boundaries may be tolerated in the practice of my invention because under the influence of the high frequency eddy currents, these tend .to segregate or ball up and thus isolated do not weaken the resulting structure substantially.- Microscopic examination of briquettes bonded together in accordance with my invention shows this balling up of impurities very clearly, but this phenomenon does not appear in comparable sam- 75 bonded by mere heat treatthorough the welding together of the'particles in internally induced currents of relatively low frequency, but for commercial work a source of cursatisfactory results.

unit time. Some advantage accrues to-employing rent having -a frequency of about 100,000 cycles per second is recommended. However, good results may be, obtained. with currents having a frequency ranging from 50,000 cycles to 200,000 cycles per second, and in general, high frequency induction furnaces of the type employed heretofore for. melting cast metal ingots, etc.,.may be employed in the practice'of my inventionwlth It-should be noted, however. that the temperature rise 'of powder metal aggregates insuch furnaces is much more rapid I than'has been obtained heretofore in melting cast metal bars and other relatively homogeneous metal masses and that it is unnecessary and in fact undesirable to raise the powder metal aggregate to a point at which substantial fusion occurs.

In the operation of my invention, powdermetal briquettes of desired configuration are' formed by compression in a mold with or without the use of a binder, and are disposed within the field of a high frequency induction coil. Alternating current, preferably of a frequency of about 100,000 Y cycles per'second, is passed through the coil and results in the induction of a plurality of high frequency eddy currentsv in-the briquette. Coherent phere or in vacuo.

masses made from the powders of any metal or combination of metals may be so treated, but the invention finds its greatest application. inpromating diffusion welding of metals which have pointsor for some other reason haveinherently lowdiffusion rates.

It is desirable to carry on the treatment-of the aggregate in anon-oxidizing or reducing atmos- This-may be done convenientlyby placing the object to be treated within arefractory container disposed within the' coil.

Proper atmosphere within the container may be' "maintained by means of a solid packing or by introducing thereinto non-oxidizing or reducing gas such as nitrogen, hydrogen, or carbon monoxide, or the container may be wholly or partially exhausted.

be'more. thoroughl understood in the light of the following detailed description of presently-prewhich shows schematically apparatus-adapted to.

ferred practices of my invention taken in conjunction with the accompanying. single figure the practice thereof.

- Referring now to the figure, it will be seen that v the apparatus comprises an'induction coiLIU connected to a suitable sourceof high frequency altemating 1 current H such, for example, as a generator "or converter. The coil preferably is in the form of a helically wound coppertube of low ohmic resistance, and water or other cooling fluid maybe passed through it; Disposed within the coil and spaced therefrom is a tubular muflie I! made of refractory and substantially non-conductive material such as quartz.

of the muilieextend'well outside of the ends of the coil, one end of the mulile being provided with hydrogen,

'The two'ends nitrogen: orcarbon monoxide may be introduced to maintain the desired non-oxidizing or reducing atmosphere within the muflle. The

other end of the, mutlie is closed by'a tightly fitting but removable plug l6. An outlet nipple I1 is provided in the plug for withdrawing gas from the muiile or for introducing a thermocouple thereinto, if desired. A briquette or group of briquettes ii to be treated is'introduced into the muflie and the plug is theninserted. Current is introduced into the induction coil and heat treatment ensues.

In the'following tests, the induction 'coil' was helical inform, made of diameter copper conductor and had approximately 15 coils about 2%" square; The length of the coil was about 6 inches. The cell was ,connected to a Lepel high frequency'spark gap converter (type 0-6), which converted ordinary commercial alternating current at220 volts and 60 cycles per second'to' current having a frequency of approximately 100,000 cycles per second for the induction coil. The input into the apparatus was approximately 6 kilowatts inmost of the tests.

Comparative test No. 1'

In this test, electrolytic iron powder having relatively clean surfaces and of a particle size ranging from to 200 mesh,'Tyler scale, was compressed-with a force of 50 tons per square inch into small bars about /a inch square and 4 inches long. The resulting bars were then Placed in the muflie in the field of the induction" coil-and current was applied. In approximately one minute at'a. temperature .of 1100 C., adequate diffusion weldingwas accomplished and the result ing metal object was sufiiciently malleable that i could be reduced to one-tenth of its original thickness by hot forging before any signs of cracking appeared.

utes, the powder consumption in this comparative test being approximately 6 k.w. Energy consumptlon in this comparative 'test was, therefore, from 30 to '60 times that tlce of my invention.

1 7 ese and other features of invention will employed in the prac- Comparative test 'No. 2

In a second test, electrolytically produced iron powder havingan average'particle size of mesh and with someiron oxide'on its surfaces was mixed with ti of relatively clean nickel powder of comparable particle size obtained from nickel carbonyl. The resulting mixture was com-' pressed into bars inch square in cross-section and 3 inches long. These bars were also placed in the muflle within the induction coil as described herelnbefore and subjected to treatment at 1100 C. forone minute. The resulting objects were sound in all particulars and resembled billets obtained by casting molten metal. Microscopic examlnation of the resulting objects indicated a high degree of homogeneity and excellent dif- "fusion. The same'degree of diffusion could not be obtained by indirect heat treatment of identical bars in a muflie in less than 2 hours The iron oxide present in the finished material was in very small amount and appeared as somewhat spherical segregations. The nickel present aided diffusion.

Comparative test No. 3

' riod of heat treatment, which adds considerably to the cost of the operation.

In the practice of myinvention, the manufacture of stainless steel by powder metallurgical practice becomes relatively simple as will be apparent from the following description.

A powder mixture having a particle size ranging from 100 to 200- mesh, Tyler scale, was made by mixing 74% by weight of iron powder, 8% by weight of nickel powder, and 18% by weight of chromium powder having a thin skin of chromium oxide on its surface. The resulting mixture was compressed under a force of about 70 tons per square inch to form bars about inch square and about 3 inches long. The bars were then placed in the muilie in the field of the induction coil and current was turned on. In 15 minutes they had reached a temperature of l350 C. as indicated by observations taken with an optical pyrometer. They were maintained at this temperature for about 1 hour. Thereafter, minutes was required to bring the muiiie and the heat treated bars to a temperature at which themuflie. During this 2-hour period, an atmosphere of dry hydrogen was maintained within the muille. Subsequently the bars were subjected to an annealing treatment in the muiile disposed in the coil. The annealing treatment consumed two hours during which the bars were heated to about 1000 0., held at this temperature for mlnutes, and then cooled to a temperature of about 30 C. when they were withdrawn from the muilie.

For comparative purposes identical bars were vplaced in a mufile disposed within an electrical resistance furnace and subjected to primary heat treatment and annealing at the same temperature as employed in the foregoing tests. The furnace employed was wound with a molybdenum resistance element and the power consumption of this element was approximately 6 law, To raise the temperature of the aggregates to a temperature 5 of 1350 C.,v required 8 hours. An additional 44 hours was required to bring about adequate bonding. Thereafter, it was necessary to let the muilie cool for 36 hours before the aggregates reached room temperature and could be removed without running risk of damage. The time consumption' for primary heat treatment in this operation was, therefore, 88 hours as compared with 2 hours by induction heating. Annealing in the resistance furnace in the mume required an additional 8 hours, so that the total operation consumed 96 hours as compared with thours to produce the same result in the practice of my invention.

The following tests illustrate presently-preferred practices of my invention with a varietyof metals:

Practice with briquettes of; iron powder Iron powders made by any of the heretofore customary methods may be employed in the practice of the invention. Thus, iron powder obaaaaeoo tained by reducing iron oxide with hydrogen or iron powder obtained from iron carbonyl is suitable. This is demonstrated by the following tests: 12 bars each inch square and about 3' inches long were made from iron powder obtained by hydrogen reduction of iron oxide. The powder was minus 150 mesh, Tyler scale, and was briquetted under a pressure of 50 tons per square inch. The resulting 12 briquettes were placed in the -muflle separated from each other by thin layers of alundum powder. The current was applied to the induction coil and in 8 minutes with a 6 kilowatt power input they reached a temperature of'approximately 1100 C. They were held at this' processes.

Four bars approximately of an inch squareand 3.inches long were made from iron powder obtained from iron carbonyl. The powder was minus 200 mesh and was pressed under a force of 70 tons per square inch. Thc'resultingbriquettes were placed in a mufile and insulated from each other by layers of alundum powder. Current .was applied to the induction coil and with a 6 kilowatt power input the bars. reached a temperature of approximately 1200 C. in 11 minutes. They were held at this temperature for 10 minutes in an atmosphere of dry hydrogen and then cooled to room temperature in the muiiie in 35 minutes. The resulting bars were solid and resembled those produced from the hydrogen reduced iron.

The resulting solid bars were ho- Practice in manufacture of binary ferrous alloys 1 Binary ferrous alloys may be produced very easily in the practice of the invention. The pres- I once of alloying ingredients, particularly nickel,

in the briquettes to be formed into alloys by diffusion welding has a favorable influence upon the treatment time, as will be apparent from the following tests:

Iron powder obtained by hydrogen reduction of iron oxide (96 parts by weight) and 4 parts by weight-of nickel powder obtained "from nickel at room temperature within the mufile in 30 minutes. These briquettes were solid, free from pores and from all standpoints could be considered ex- Current was then turned off and the briquettes cooled cellent. It was found, however, that the same results could be obtained with an even lower period of heat treatment. The following test was comparable in all respects to that described immediately-above, except that after an 8 minute heating period the briquettes were held for only one. .minute at 1100? C. This time was suflicient to accomplish a thorough bonding together ,of the particles and microscopic examinationof the resulting bars showed them to be as satisfactory as vthose obtained by holdingthe briquettes for 10 minutes at 1100 C.

" Practice in manufacture cf ternary iron alloys That the bond obtained'between particles in the practice of my invention isstrong, is indicated by the fact that stainless steel 'made in the practice of my invention maybe subjected to a waterreduced iron powder, finely-divided low carbon ferrochromium containing 70% chromium, and

, nickel powder obtained from nickel carbonyl. The size of the powder ranged from 100 to 200 mesh. 2

It was mixed in dry condition for 12 hours, then pressed into briquettes under a force of 70 tons per square inch and placed in a muflle under the influence of the high frequency currents and in an atmosphere of dry hydrogen. In 12 minutes, with a power input of 6 kilowatts, the'briquettes attained a temperature of 1375 C. at which temperature they were held for one hour. They were.

then allowed to cool in the muille for 40 minutes and at room temperature were repressed under a force of '70 tons per square inch. Following repressing" they were annealed in the muflle for 9 minutes wherean input of .6 kilowatts raised the temperature of the repressed" bars to approximately 1000 C. Thebars were held at this temperature for one hour in an atmosphere of dry hydrogen and 'they were withdrawn from the muille and quenched immediately in water.

In another test, the conditions were identical with those described immediately above except in the following particulars: The heat treatment took place in a vacuum. The pressure within the mufiie was kept under 1 mm. of mercury andthe cooling after the first heat treatment consumed furnace for one hour in an atmosphere of carbon monoxide and. carbondioxide. From the high temperature (approximately-1000 C.) prevailing at the end of the annealing period, the bars were cooled immediatelyto room temperature by water quenching.

In both of these tests, the barssufiered no ill efiects in the water quenching and were entirely satisfactory from all-metallurgical standpoints. Practice with gum-ferrous metal briquettes The bonding together of non-ferrous metal briquettes in .the practice of my invention is well to a temperature of 600 C.-in- 4 minutes with a.

gen. At the end of the 10 minute period, current was turned on the induction coil and the bars were allowed to cool in the mufile for 15 minutes, at the end of which time they were approximately atv room temperature. These bars, like-all the others described herein, :were entirely satisfactory.

Twelve copper bars of the. same size as those made. from'aluminum were made fromelectrolytically-produced copper powder of minus 150 1 mesh under a pressure 0'! 50 tons per square inch.

temperature they were held ior-5 minutesin a" dry hydrogen atmosphere and, of course, in the presence of the induced eddy currents. They were then allowed to cool for 20 minutes. Microscop-ic examination of the bars thus formed showed them to be comparable-in structure to bars made by heretofore customary'casting operations, except that the non-metallic impurities such as copper oxide were more segregated and notso, 4

completely distributed throughout the Mahufacture of non-ferrous qly'ects Twelve bars containing 90% copper and 10% tin were niadeas'follows: j

powder and copper powder of a. mesh size ,minus 150 were mixed in th'edry statefor 8 hours.

and then pressed into briquettes about of an inch square and 3 inches long under a pressure of. 50 tons per square inch. The resulting V I briquettes were heated as described hereinbefore e in the muifle for 5 minutes with a power; input of 4.5 k w. In the five minutes the briquettes attained a temperature of 725 C. and they were heldat this. temperature for 10 minutes inthe -atmosphere prevailing in-themufile, namely, one

' of dry hydrogen; At the end of 10 minutes, the,

current was turned ofiand the resulting bronze bars were allowed-to cool for 20 minutes'to ap-' proximately room temperature, when physical tests and'microscopic examinations were made. Results were excellent. V

In another test; similar bars were raised in 6 this point for '10 minutes. It was found, however,

that the bars thus treated were not superior to metal between ;the powder particles by inducing alternating currents a frequency of at which'comprises' accelerating the difiusion of minutes to a temperature of .825 C. and held at which metal pow er particles v least 50,000 cycles'per second within the 'mass while maintainingthe mass in a relatively supported condition at an elevated temperature 2. In a process in which metal powderv particles are pressed into a coherent mass which is subjected toheat treatment-to bond, the particles together, the improvement which comprises heating said mass by inducing eddy currents therein and maintaining said.mass ma relatively unconfined condition at: an elevated temperature but below the 'melting points of its metallig constituents while maintaining therein eddy currents having a frequency oLat least 50,000cycles per second, whereby diffusion of the metal between powder particles the particles is accelerated! 3. Ina process'in. which metal areformedinto a coherentmass and-bonded together' by heat treatment, the improvement which 'comprises'subjecting such a mass containing metal particles having skins of solid oxidation products. thereon in a relatively. unsupported con. -ditionto the action of highfrequency eddy curreaches a temperature ranging between the rents induced within the mass imtll the mass metallic: constituents of the mass and a temperature that is about two-thirds of said weighted average and maintaining. the mass in said tem-.

perature range in said relatively unsupported condition while inducing eddy currents having a frequency of from 50,000 to 200,000 cycles per second therein until the particles have become bonded together, whereby diffusion welding of the particles is accelerated and the distortion of the" 50,000 to 200,000 cycles per-second within the 7 mass while maintaining the mass in arelatively unconfined condition at a temperature below the melting point of any of its metallic constituentsbut above a temperature that is about two-thirds of the weighted average of the melting points of the metallic constituents of the mass.

5. In a process in which a coherent mass of metal powder particles is subjected to heat treatment to causethe particles to bond together, the

- improvement which comprises forming said mass of metal powders having films of gas and solid oxidation productsuponthem so that the particles in the mass are at least partially insulated from each other, and heating the mass in a relatively unsupported condition thus formed to a point in a range below the weighted average of the metallic constituents in the mass but above a temperature that is about two-thirds of said weighted average by inducing eddy, currents having a frequency, of at least 50,000 cycles per second within the mass, and maintaining said mass at atemperature within said range while inducing said currents therein until the particles become bonded together.

6. In a process in which a coherent mass of metal powder particles is subjected to heat treatment to cause the particles to bond together, the improvement which comprises forming said mass of metal powders having solid oxidation products of the metallic constituents of the powder upon the surfaces thereof so that the particles in the mass are at least partially insulated from each, other and heating the mass thus formed to a point ina range below the'weighted average of the metallic constituents in the mass but above a temperature that is about two-thirds of said weighted average by inducing eddy currents having a frequency ranging from 50,000 to 200,000 cycles per second within the mass, and maintaining said mass in a relatively unconfined condition at a temperature within said range while inducing said currents therein until the particles become bonded together.

'7. In a process in which metal powder particles having solid non-metallic constituents on the surfaces thereof are formed into a coherent mass and subjected to heat treatment to bond the particles together, the improvement which comprises coalescing and segregating the solid non-metallic constituents in said mass to facilitate difiusion of metal between the powder particles by inducing in said mass high frequency eddy currents having a frequency ranging from,

50,000 to 200,000 cycles per second while maintaining the mass in a substantially unsupported condition at an elevated temperature but below the melting point of any of its metallic con- I stituents.

8. In a process in which a bronze object is formed by heat treatment of a coherent mass of metal powder containing tin and copper, the improvement which comprises inducing within such a. mass containing metal particles having skins of solid non-metallic impurities thereon high frequency eddy currents having a frequency of at least 50,000 cycles per second and of an intensity sufiicient to raise the temperature of the mass -to a temperature of about 725" C., and maintaining in the mass at about said temperature in a relatively unconfined conditioniwhile inducing said currents therein for a relatively short time. r

9. In a process in which stainless steel is formed by heat treatment of a coherent mass of metal powder containing iron, nickel, and

chromium, the improvement which comprises inducing within such-a mass containing metal particles having thin skins of solid oxidation products thereon high frequency eddy currents having a frequency of at least 50,000 cycles per second and of an intensity sufiicient-to raise the temperature 'of the mass to about 1350 C., and maintaining the mass at about said temperature in a relatively unconfined condition while inducing said currents therein for a relatively short time.

10. In a process in which a coherent mass of metal powder particles containing chromium powder having a skin of oxidation products on the surface thereof is subjected to heat treatment to bond the metal powder particles together, the improvement which comprises heating said mass to an elevated temperature but below the melting point of chromium by inducing currents of a frequency of at least 50,000 cycles per second 

